Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.
A dye is a colorant which is molecularly dispersed or solvated by a carrier. The carrier can be a liquid or a solid at room temperature. A commonly used carrier is water or a mixture of water and organic co-solvents. Each individual dye molecule is surrounded by molecules of the carrier. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor light-fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness.
In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers which serve to keep the pigment particles from agglomerating and settling out of the carrier. Water-based pigmented inks are prepared by incorporating the pigment in the continuous water phase by a milling and dispersing process.
U.S. Pat. No. 5,679,138 relates to a process for preparing an ink jet ink containing nanoparticles of organic pigments. This process employs milling media of polymeric resins such as cross-linked polystyrene. There is a problem with using this media, however, in that the jetting reliability is not as good as one would like it to be.
U.S. Pat. No. 5,755,861 relates to a process for preparing an ink jet ink which has an improved jetting reliability by removing trace metal impurities from the ink. The metal impurities are removed from the ink by a cation-exchange treatment using a cation-exchange resin. There is a problem with this process, however, in that it requires that the ink be passed through a column packed with an ion exchange resin which adds another step and expense to the process.
It is an object of this invention to provide a process for preparing an ink jet ink having an improved jetting reliability. It is another object of the invention to provide a process for preparing an ink jet ink which will remove trace metal impurities without requiring a treatment with an ion exchange resin.